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Pre Implantation Genetic Diagnosis: What It Is (And Isn’t) Actually Capable Of

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There is a lot of talk about “Designer Babies”. Everybody knows that it is already possible to screen embryos for certain genetic diseases, so how far could we possibly be away from the process of picking and choosing specific traits for our babies, having them designed according to our wishes, have the modified embryo implanted, and nine month later giving birth to exactly the baby we wanted? Actually, can we even be sure the rich and powerful aren’t already secretly doing just that in shady hidden labs, run by megalomanic mad scientists? Are there already designed people walking among us?

The short answer is no. The technique used for avoiding genetic diseases in couples that have a high risk of giving birth to genetically impaired offspring is called Pre Implantation Genetic Diagnosis, usually shortened to “PGD” or “PIGD”.
Quite unlike what most people imagine, this doesn’t actually involve sequencing the embryo’s DNA: There is no doctor looking at thousands of lines of genetic code and saying stuff like: “it’s gonna be a tall blonde boy with an IQ of 126. Do you want us to remove his predisposition to alcoholism?”
In reality, PGD is very limited. The embryos are scrutinized at a stage when they consist of 8 cells. In this stage, all of the cells are still pluripotent (they haven’t specialized to be anything specific yet) and the embryo is still capable of being frozen and later reanimated without taking any damage. From these 8 cells, two are removed for the purpose of screening. That doesn’t impair the embryo, as at that stage the remaining six cells keep dividing into perfect copies of each other and the growth simply continues.
The two removed cells each contain two sets of chromosomes. That is very little genetic material to work with.

Two copies of each of the 23 human chromosomes.

Human DNA

With so little material, you have to know very specifically where to look and what you’re looking for in order to be able to spot anything at all. Sequencing the complete genome is not possible under these conditions. So “Screening” is limited to just that: Tests that check for a very specific genetic abnormalities in very specific places on the genome. In a healthy organism, every chromosome ought to be found twice in each cell. Some diseases are caused by one of these copies missing (monosomy), or there being three rather than two copies present (trisomy). These are the kind of genetic defects that we can spot with PGD. And that’s great, because it allows us to avoid some pretty horrible diseases. But all the hype is exaggerated: All those who are panicking because they think that if we don’t outlaw PGD, people will be able to walk into a clinic with a list of attributes they’d like their child to have, and be furnished with an accordingly designed embryo ready for implantation, have no idea what they are talking about.

Anything beyond recognizing the most obvious of oddities is still very much science fiction.

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